C Series

The C series of paper is used only for envelopes; as the area of a C series sheet is the geometric mean of the A and B series sheets of the same number. This means that an A4 sheet is slightly smaller than a C4 sheet, which in turn is slightly smaller than a B4 sheet. In practice this means that an A4 letter could fit unfolded into a C4 envelope which could then fit into a B4 envelope.

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A Series Paper Sizes
A4 Paper Size
US Letter Paper Size
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ISO Envelope Sizes
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